Why 29 days in February? The Roman calendar mess
From Numa to Caesar to the Gregorian reform — and why this means "year to day" conversions have caveats.
Converting years to days is tricky, and the reason is historical.
The Roman calendar (legend: Romulus, 8th-century BC) had 10 months, 304 days. February was added later by King Numa Pompilius, who also introduced January.
To make the year align with the lunar cycle (12 × 29.5 ≈ 354 days), Romans added a leap month every few years. Caesar's reform made the year exactly 365 days with a leap day every 4 years. Augustus refined it.
The result: a calendar year is 365 days (or 366 in a leap year). A Julian year is fixed at 365.25 days. A tropical year (the time for the sun to return to the same equinox) is 365.2422 days.
When you see "year to day" on this site, we use the Julian year (365.25 days, exactly 31,557,600 seconds). It's the SI standard. It's also why the same conversion gives 365.25 days, but if you typed "how many days in 2026" you'd get a real-world answer of 365.
For most purposes the difference (0.0078 days per year ≈ 11 minutes) is irrelevant. For leap-second calculation, satellite trajectories, and historical chronology, it isn't.